Neurophysiology

Membrane Potential - Resting membrane potential from separation of opposite charges across membrane (i.e. –ve

inside vs +ve outside)

o Conc. gradients maintained by K+/Na+/ATPase fixed chemical driving forces of

ions

▪ Inside: K+, large organic anions

▪ Outside: Na+, Cl-

o Ion leakage channels ions diffuse through according to chemical + electrical

driving forces imbalance of charge membrane potential

▪ Conc. gradient doesn’t change significantly due to large no. of ions

o Usually ~ -60mV (reference to 0 mV outside)

- Non-gated ion channels/leakage channels allow ions (K+, Na+) to flow down electrochemical

gradient

o Change in membrane potential w/o significant change in conc. gradient

o Electrochemical driving force: Adding chemical + electrical driving force vectors

o Chemical driving force: Due to conc. gradient

o Electrical driving force: Due to differences in charge

- Forces and movement of K+ without electrical potential: Equal charges both inside and

outside membrane

o Chemical driving force outside

o Movement of one K+ outside anions > cation inside -ve inside (relative to 0)

-ve membrane potential

o Growing electrical force inside (attracted to –ve)

o If K+ is allowed to continue diffusing outwards, equilibrium is established:

▪ Chemical = electrical driving force no net force

▪ No movement of K+

- Forces and movement of Na+ without electrical potential: Balanced charges inside and

outside

o Chemical driving force inside

o Movement of Na+ inside anions > cations outside +ve inside (relative to 0)

+ve membrane potential

o Growing electrical force outside

o If Na+ continues diffusing inwards, equilibrium is established w/ chemical = electrical

driving force

- Nernst potential, E: Potential at which there is no net movement of ion; specific to particular

ion assuming membrane only permeable to single ion

o Ek = -75 mV

o ENa = +55 mV

R = Rydberg’s gas constant, T = temperature, Z = charge of ion, F = Faraday’s

constant

o Higher temperature higher thermal energy of ion higher chemical driving force

higher magnitude of Nernst potential

- Resting membrane potential closer to Ek because of higher permeability (20 times) to K+

ions than Na+

o Goldmann equation: Determination of membrane potential using conc. of all ions

(inside and outside) and permeability of membrane to ions

- Non-gated ion channels buffer resting membrane potential

o Increase in intracellular [Na+] membrane potential becomes less –ve (-60 mV -

55 mV)

▪ Lower electrical driving force of K+ and Na+ inside (inside less –ve)

▪ Higher net flow of K+ outside (less opposing electrical force), lower net

flow of Na+ inside (less chemical and electrical force)

▪ Repolarisation of membrane as outside becomes more +ve & inside

becomes more –ve

Action Potential

- Depolarisation = Inflow of +ve ions e.g. Na+ Membrane potential increases above resting

potential

- Repolarisation = Outflow of +ve ions e.g. K+ after depolarisation Membrane potential

decreases to return to resting potential

- Hyperpolarisation = Outflow of +ve ions continues after repolarisation due to slow closing of

VGPC Membrane potential decreases below resting potential

- Return to Vm after hyperpolarisation = Buffering by leakage channels

Factors Affecting Depolarisation + Conduction Velocity - Electrotonic conduction: Passive spreading of depolarisation down membrane; does not

involve voltage-gated ion channel to amplify current like AP

- - Depolarisation decreases in amplitude as distance from initial stimulus increases due to

weaker local circuit currents

o Membrane conductance w/ ion channels: Ions leak out

▪ Charge flows out and returns to original stimulus site

▪ Greater diameter greater SA more ion channels higher membrane

conductance

o Axoplasmic resistance (fluid inside axon only semi-conducting): Resists depolarising

current flowing through axon

▪ Thinner axon higher longitudinal resistance more current prevented

from flowing more current leaks out rather than flowing down

- Conduction velocity inversely proportional to electrical capacitance and axoplasmic

resistance

o Electrical capacitance (storage of charge) of lipid bilayer membrane

▪ Charges accumulate on both sides of insulating membrane

▪ Currents must neutralise/recharge sides of membrane takes longer to

depolarise/repolarise membrane

• Non-linear depolarisation

• Takes longer for current to move down membrane

▪ Thinner membrane greater electrical capacitance per square mm of axon

membrane as charge is separated by less distance (greater attraction)

▪ Greater diameter i.e. thicker axon greater SA greater total capacitance

o Axoplasmic resistance: Thinner higher resistance slows down current (& less

current flows down as more flows out)

Action Potential - Self-propagating event consisting of depolarisation and repolarisation of membrane

- Threshold value = Membrane potential at which AP is triggered

o AP is ‘all-or-none’; amplitude of AP does not depend on amplitude of initial stimulus

o Threshold differs for different neurons, depending on:

▪ Density of voltage gated sodium channels: Greater density less diffusion

of charge between VGSC activation of VGSC even w/ lower membrane

potential lower threshold potential

- Threshold voltage = Stimulus voltage needed to trigger AP

o Depends on diameter of axon; thinner higher axoplasmic resistance higher

threshold

- Significantly high depolarisation (> threshold value) AP

- Graded potentials (sub-threshold) can sum AP if they are close enough

Phases 1. Voltage gated sodium channels (VGSC) open when membrane depolarises to threshold

a. Resting state = closed activation gate

b. Activated = activation gates open quickly Na+ diffuses in

c. Hodgkin cycle: More VGSCs open w/ greater depolarisation (+ve feedback)

d. Inactivation: Inactivation gate slowly inactivates VGSC; Na+ cannot pass through

regardless of stimulus strength

i. Most inactivated at peak of AP

ii. absolute refractory period

2. Voltage gated potassium channels repolarise

a. Resting state = closed

b. Activated = activation gates slowly open after VGSC open; more open w/ greater

depolarisation (+ve feedback)

c. Closing: activation gates slowly close

APs Don’t Change Conc. Gradient - Movement of few (1/1000000) ions carry significant charge change membrane potential

- Inhibition of Na+/K+/ATPase pump (which maintains resting potential):

o Can still generate AP resting membrane potential not differing greatly

o ENa and EK don’t change for a while (~20 mins) Conc. gradient doesn’t run down

Methods of Propagation of AP

Continuous Propagation in Unmyelinated Axons (Slow) - AP stimulated locally and propagates down via Hodgkin’s cycle i.e. amplification of

depolarisation by VGSC

- Very slow due to needing to open and close VGSC

- Only travels in one direction due to hyperpolarisation in previously depolarised sites

Saltatory Propagation in Myelinated Axons

- Schwann cell (in periphery) wraps around axon multiple times myelin sheath (multiple

layers of Schwann cell membrane)

- - Node of Ranvier: Section of unmyelinated axon membrane between myelin

o High density of VGSC lower threshold potential to generate AP w/ less diffusion

and loss of current between VGSC

o Hodgkin cycle triggered at each node amplification of signal

- Effects of myelin on AP propagation:

o Increases CV by only triggering AP’s at nodes (jumping) – maximises electrotonic

conduction i.e. passive diffusion of charge w/o VGSC etc.

▪ Ion leakage channels covered higher transmembrane resistance/lower

conductance less transmembrane leakage local currents stronger and

flowing further

▪ High density of VGSC’s at nodes lower threshold AP

o Increases diffusion CV w/ lower capacitance

▪ Greater separation of extracellular fluid & axoplasm lower membrane

capacitance depolarising current is quicker as does not need to neutralise

charge

- Demyelination:

o Increased outflow of current through leakage channels weaker local currents

lower amplitude of depolarising current

o Higher membrane capacitance slower depolarisation at membrane, slower

movement of charge down axon

Stimulus Intensity and Codes

Frequency Code - Higher stimulus intensity higher AP frequency in same neuron (‘spikes’ of AP in a ‘train’ of

many)

o As depolarising current is strong enough to surpass threshold multiple times

- Limits of frequency code: AP frequency may not differentiate intensity differences because

o Absolute refractory period: VGSC are all inactivated, no Na+ can pass through

next AP impossible

o Relative refractory period: VGSC are closed, VGPC are open hyperpolarisation w/

membrane potential < resting potential more intense stimuli needed for next AP

Population Code - Higher stimulus intensity more nerve fibres within nerve generate AP

- Shown in CAP recording w/ increased amplitude of CAP

- Limits:

o Amplitude of CAP only shows sum of most common speed fibres

Recording Compound Action Potentials (CAP) - Applying stimulating current to nerve AP in axons (not always all of them)

- Extracellular recording electrodes measure current

- AP in different axons propagate at different rates

- CAP = sum of depolarisation at the surface of nerve (consisting of many axons)

o Fast propagating AP contribute to start of CAP

o Slow propagating contribute to end of CAP

- Latency = time between stimulus and recording of AP

o Measured from start of stimulus artefact (electrical conduction – not due to nerve)

o Due to time taken to generate AP, and time for AP to propagate down axon to

recording electrode

o 2 latencies – one to onset (fastest fibres) and one to end (slowest fibres)

- Biphasic: CAP has +ve and –ve deflection due to distance between the two recording

electrodes movement of charge between (as potential difference measured by difference

between the two sites)

- Monophasic: Generated by crushing nerve between two recording electrodes

- Types of stimuli:

o Subthreshold: No AP generated

o Threshold: AP generated

o Maximal: All nerve fibres generate AP

o Supramaximal: Stimulus intensity > maximal

Chemical Synaptic Transmission at Neuromuscular Junction

Mechanism

1. AP depolarises nerve terminal

2. Voltage gated Ca2+ channels open Ca2+ influx into terminal

3. Increase in Ca2+ conc. exocytosis of chemical neurotransmitter acetylcholine (Ach) into

synaptic cleft (due to Ca2+ sensors on vesicles)

4. Ach binds to nAch receptors (ligand gated cation channels) in postsynaptic (basement)

membrane

a. Na+ influx > K+ efflux due to electrochemical gradient

5. Na+ influx into postsynaptic membrane endplate potential (EPP)

6. Sufficient EPP AP w/ opening VGSC that propagates down muscle

Endplate Potentials (EPP) - Recording:

o Stimulating electrodes on axon of motor neuron

o Recording electrodes: Intracellular electrode inside muscle fibre cell, extracellular

reference in extracellular fluid

- Triggered by nerve impulse

- Amplitude of EPP depends on extracellular [Ca2+]e

o Affects influx of Ca2+ into nerve fibre affects amount of Ach released

- If EPP > threshold, AP triggered in muscle fibre

- Usually suprathreshold as AP in nerve fiber ~100 quanta of Ach released sum of ~100

mEPP large depolarisation ~70mV AP

Miniature Endplate Potentials (mEPP) - Occurs spontaneously due to release of 1 vesicle of Ach

- Small amplitude (~0.5 mV)

- Amplitude of mEPP independent of [Ca2+]e as not calcium induced release of Ach -

spontaneous

Release of Ach in Quanta - Amplitude of EPP are always discrete value of amplitude of mEPP

- Ach released in vesicles w/ specific amount (quanta) (~5000) in each (consistent size of each

vesicle)

- Quantal content (no. of quantas released) depends on:

o [Ca2+]e in synaptic cleft; alters chemical driving force of Ca2+ [Ca2+] increase

o Probability – Failure may occur w/ no quanta released in response to nerve impulse

- EPP amplitude = quantal amplitude x quantal content

o Quantal amplitude = Amplitude of EPP from release of single quantum

Acetylcholine Recycling 1. Ach is broken down by acetylcholinesterase in synaptic cleft acetate + choline

2. Choline is transported back into axon via secondary active transporter

3. Choline acetyltransferase forms Ach using acetyl coA + choline

4. Packaging of Ach via secondary active transporter

- Hydrolysis/reuptake prevents Ach from binding to AchR limits amplitude and duration of

EPP

- Fast: Ach exocytosis, activation of AchR, Ach hydrolysis by AchE

- Slow: Ach synthesis, packaging (in vesicles) and trafficking, Ch reuptake into nerve terminal

- High nerve stimulus intensity high frequency of AP generation high frequency synaptic

transmission decrease in quantal content (less quanta of Ach released) as recycling

cannot keep up decrease in EPP amplitude

- *Myasthenia gravis: Reduction in AchR lower EPP fewer AP generated cannot

generate muscle contractions

o Can be relieved by inhibiting AchE less Ach hydrolysis increased binding of Ach

to AchR

Synaptic Transmission in CNS (Between Neurons)

Mechanism 1. AP depolarises nerve terminal

2. VGCC open increase in [Ca2+]

3. Increase in [Ca2+] exocytosis of NT via fusing of vesicles to membrane

4. Binding to NT receptor response to NT

a. Opening of ligand-gated ion channels depolarisation/hyperpolarisation of

membrane

b. Receptor-enzyme enzyme activation/inactivation kinase/second-messenger

pathway altered excitability, changing protein production etc.

- Presynaptic bouton: Swellings at terminals of axons

o Varicosity: Bouton en passant – swellings along axon branches.

- Post-synaptic spine: Swellings on dendrites.